Loudspeaker

ABSTRACT

Conventional analog loudspeakers have a limited dynamic range as compared to the available dynamic range of digital recordings. Digital recordings use up to 24 bits and this implies a dynamic range of 141 dB. Digital loudspeakers, involving 2 N  single bit devices (with N=24, this number is 1.7×10 7 ) have been proposed. The present improvement is the provision of at least one loudspeaker, a plurality of analog drivers and the audio input supplied to a control processor which in turn drives one or more of the plurality of independent analog drivers. The number of drivers in operation at any one time is determined by the amplitude of the input audio signal to the control processor.

This application is the U.S. national phase of international applicationPCT/GB02/00483 filed 4 Feb. 2003, which designated the U.S.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to loudspeakers and in particular to loudspeakerswith improved dynamic range as compared to existing loudspeakers.

2. Discussion of Prior Art

Conventional (or single input) loudspeaker systems can be defined assystems in which the master drive signal may be passed to a plurality ofdrivers, but for which, at any particular frequency the relationshipbetween the signals passed to each driver is fixed. A driver in thiscontext could mean an electro-magnetic induction coil (as used inconventional loudspeakers) or a piezo-electric pad or any other devicethat can cause a panel-form loudspeaker or a loudspeaker cone to move.

FIG. 1 shows a conventional loudspeaker system comprising threedrivers/loudspeakers 1, 2, 3. A master signal 4 is split by filters 5, 6and 7 (high pass filter, band pass filter and low pass filterrespectively) into three frequency ranges, treble 5 a which goes tospeaker 1, mid-range 6 a which goes to speaker 2 and bass 7 a which goesto speaker 3. This represents a multiple speaker system in which thereis a frequency split of the main master drive signal 4. The relationshipbetween each of the drivers 1, 2 and 3 is fixed and is not dependent onthe level of the master signal.

Conventional analogue loudspeakers have a limited dynamic range ascompared to the available dynamic range of the latest digital recordings(for example 24 bit or DSD). Digital recordings use up to 24 bits andthis implies a dynamic range of 141 dB. Digital loudspeakers, involving2^(N) single bit devices (with N=24, this number is 1.7×10⁷) have beenproposed—see WO96/31086. However, these suffer from obvious complexityand poor performance associated with the interaction effects between thedifferent devices, which have discouraged widespread use of suchsystems. A further problem is the inability of most loudspeakers toreproduce realistic absolute levels of sound (up to say 120 dB at 1 mwithout distortion), so such digital loudspeakers cannot take fulladvantage of the 24-bit fidelity.

A conventional loudspeaker system, such as that shown in FIG. 1, willsuffer distortion and other detrimental effects if the dynamic rangesupplied to any of the drivers/loudspeakers 1, 2 or 3 exceeds much morethan 100 dB. Note, although conventional speakers can be constructed tohave a dynamic range of approaching 120 dB they are very expensive. Moreusually the dynamic range of a conventional speaker is in the region of100 dB.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide aloudspeaker system, which overcomes or at least mitigates theabove-mentioned problems with prior art systems.

Accordingly this invention provides a “multiple input loudspeakersystem” (as herein defined) comprising one or more loudspeakers and aplurality of analogue drives arranged in use to drive the one or moreloudspeakers wherein, in use, the one or more loudspeakers are input adrive signal having a time varying signal level and, at any particulartime, the signal level measured at the input to the loudspeaker systemdetermines the operational state of each of the drivers.

A “multiple input loudspeaker” may be made up from a plurality ofconventional analogue loudspeakers or alternatively from a panel-formloudspeaker, sometimes referred to as a flat panel loudspeaker or amulti-mode radiator, having a plurality of analogue drivers.

A “multiple input loudspeaker” is not a conventional multiple channelloudspeaker system (used for example in surround sound or stereo soundsystems) although it could be applied to such a multiple channel system.

“Multiple input loudspeaker” systems in contrast may be defined assystems in which a master drive signal is devoted into a plurality ofsignals which are applied to a plurality of drivers but for which at anyparticular frequency, the relationship between the signals passed toeach driver depends on the level of the master signal. This distinctionis illustrated in FIGS. 1 and 2.

The plurality of analogue drivers can be connected to conventionalspeakers or more conveniently the plurality of drivers can drive asingle panel-form loudspeaker.

Panel-form loudspeaker technology is able to take advantage of digitalfidelity because it is able to inherently produce very high absolutelevels of sound. By using a panel-form loudspeaker combined with aplurality of analogue drivers or exciters it is possible to overcome theproblems of complexity, interaction effects and loudness which limit thebenefits of existing solutions. Prior art devices have suggested the useof more than one driver for a single loudspeaker, but none of them haverecognised the need to control how these drivers interact to obtain thebenefits of the present invention.

Since the loudest elements of music signals tend to occur at the lowestfrequencies, the width of the window can be chosen to properly producethe necessary low frequency signals whilst avoiding rapid changes ingain to each loudspeaker.

Preferably, at very low levels only one driver is activated and at veryhigh levels all drivers are activated, and the sum of all the driveroutputs equals the required signal outputs at all times.

DETAILED DISCUSSION OF EMBODIMENTS

At low frequencies the acoustic pressures produced by the action of eachactive driver will tend to add in a linear fashion. In order to ensurethat the combined output from all drivers is correct a control signalcan conveniently be applied to the linear time signal to maintain thesum of the linear time output equal to the required signal output.

In contrast, at high frequencies the acoustic pressures produced by theaction of each active driver will add in a power manner. Therefore inorder to ensure that the combined power output is correct a controlsignal can conveniently be applied to a suitable squared time signalsuch that the sum of the acoustic power output is equal to the desiredpower output. This is beneficial at the higher frequencies where driverstend to act independently of one another.

Preferably, the controller operates in both linear and power signals,such that at low frequencies the controller maintains the linear sum,whilst at high frequencies the controller maintains the power sum. Thisarrangement covers a wide frequency range.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the loudspeaker system according to the present inventionwill now be described with reference to the accompanying drawings inwhich:

FIG. 1 illustrates a conventional multi-channel loudspeaker system;

FIG. 2 illustrates a multiple-input loudspeaker system according to theinvention;

FIG. 3 illustrates an algorithm (=algorithm 1) to control operation of aloudspeaker according to the present invention.

FIG. 4 illustrates an algorithm (=algorithm 3) to control operation of aloudspeaker according to the present invention;

FIG. 5 illustrates the sliding boxcar averaging process to determine thecontrolling master amplitude, according to the present invention;

FIG. 6 is a block diagram of a panel-form loudspeaker in accordance withthe present invention;

FIG. 7 illustrates one example of a radiator and drivers for apanel-form loudspeaker in accordance with the present invention;

FIG. 8 illustrates another example of a radiator and drivers for a panelform loudspeaker in accordance with the present invention;

FIGS. 9 and 10 illustrate a suitable smoothing function (for use withalgorithm 3) to apply to each driver such that new drivers are broughtin smoothly; and

FIG. 11 illustrates an alternative structure of panel-form loudspeakerin accordance with the present invention.

Note: throughout all the Figures like numerals are used to denote likefeatures.

FIG. 1 as described above represents a conventional loudspeaker system.FIG. 2 shows a multiple input loudspeaker system covered by theinvention comprising a number of drivers 10 a, 10 b, 10 c, 10 d . . . 10n, which receive their input from a master signal 8. Note, this mastersignal could be the same as master signal 4 in FIG. 1 or it couldrepresent one of the channels 5 a, 6 a or 7 a or any other aspect of anaudio system.

Each master signal 8, see FIG. 2, is a time varying data steam, and itis this varying amplitude level that determines the signal sent to eachdriver 10 a . . . 10 n. By choosing suitable factors in the calculationof the drive signals for each driver it is possible to make sure that nodriver is overloaded and each will operate within its linear dynamicrange with low distortion. Irrespective of the choice of loudspeaker(i.e. panel-form or conventional) there are a number of alternativealgorithms by which the analogue drivers can be controlled.

In a first algorithm, an oversampling method is used. The signal to eachdriver is determined at each digital data point using INT {(x+k)/n} forthe kth driver, 0≦k<n, where x is the basic signal level expressed as asigned integer, n is the number of drivers and INT{ } implies the lowestinteger part of. This algorithm is shown in FIG. 3 for a full level sinewave with 16 drivers. This algorithm is complex, but overcomes mostproblems associated with the use of conventional loudspeakers fordigital recordings, because all drivers are always activated and alldrivers use substantially the same waveform as shown in FIG. 3.

Alternatively, in a second algorithm, a first driver is activated anddriven until the signal level reaches a first predetermined level; asecond driver is activated when the signal level reaches the firstpredetermined level; and subsequent divers are activated as the signallevel reaches subsequent respective predetermined levels; whereby allactivated drivers share load equally at all activated levels.

Alternatively, in a third algorithm, a first driver is driven until thesignal level reaches a first predetermined level, wherein a seconddriver is activated as the signal level reaches the first predeterminedlevel; wherein subsequent drivers are activated as the signal levelreaches subsequent respective predetermined levels; whereby each newlyactivated driver takes the load required and all other activated driversare saturated. This algorithm is shown in FIG. 4 for a full level sinewave with 16 drivers.

For Algorithm 1 all drivers are activated at all signal levels.Algorithms 2 and 3 have the advantage that at low signal levels only asingle driver is activated, thus potentially giving higher quality soundat such levels than would be the case with algorithm 1. Algorithm 3 hasthe advantage of only having signal gradient discontinuities at thechange over levels—thus reducing unwanted transient switching problems.

Preferably for algorithms 2 and 3, an exponential or other smoothingfunction is applied to the control signal for each newly activateddriver such that the addition of a new driver to all the other activateddrivers is achieved in a continuous manner.

Algorithms 2 and 3 can be considered as producing drive signals witheffective time-varying gain. However, rapid changes in the gainassociated with each driver can cause undesirable non-linear distortioneffects and therefore a still further way of controlling the drivers isto control the rate at which the gain to each driver changes so that itis changed in a smooth fashion. Therefore, preferably, a smoothingfunction is first applied to the master drive signal at the input to theloudspeaker. The smoothed drive signal can then be used to calculate thenumber of operational drivers required.

A window, such as a sliding boxcar, can be employed successfully in this“smoothing” role. Whereby, the gain applied to each driver is based onthe weighted average signal measured as the mean across a number ofsamples which encompass points both in the future and the past, relativeto the current time sample of the master drive signal. Thus, for anytime t, the gain is calculated from a weighted mean signal between thetimes t−mΔt and t+nΔt, where Δt is the time between individual signalsamples and m and n are integers. These integers may be equal or may bechosen to favour either the past or future portions of the signal. Thetotal duration of the window (m+n) Δt effectively controls the rate atwhich the gain to each driver changes. This smoothing box-car functionis illustrated in FIG. 5 wherein an initially rapidly changing signal inFIG. 5 a is smoothed by the action of the box car function into thesmooth signal of FIG. 5 b.

FIG. 6 shows one example of a panel-form loudspeaker according to thepresent invention. A signal, for example from an amplifier (not shown)is input to a control processor 11. The output of the control processor11 modifies the operation of one or more drivers 10, which are attachedto a radiator panel 12 and when operated excite a multi mode response ofthe panel (Note: this arrangement is equivalent to the one shown in FIG.2, i.e. there are a number of drivers 10 a . . . 10 n. The onlydifference is the speaker technology used, conventional speakers in FIG.2 and a panel form loudspeaker in FIG. 6).

The panel is provided with a plurality of drivers, which are arrangedacross the panel. The arrangement of multiple drivers aims to excite allmodes of the panel and to avoid interactions with each other. This canbe achieved using a spiral starting just off-centre or an irregularpattern, both producing driver locations spread throughout the panel.Alternatively, drivers may be arranged in a more regular manner, eitherconcentrated at the centre or spread across the panel. This is stilleffective because the panels themselves tend to be slightly irregularwhen manufactured. FIGS. 7 and 8 illustrate two arrangements of multipledrivers, although others are possible.

In use, the panel-form loudspeaker of the present invention is operatedby the control processor comparing the input or base signal with a setof known criteria and then controlling the operation of the drivers inresponse to this. For example, an oversampling method can be used. Thesignal to each driver 10 is determined at each digital data point usingINT{(x+k)/n} for the kth driver, 0≦k<n, where x is the basic signallevel expressed as a signed integer, n is the number of drivers and INT{} implies the lowest integer part of. This algorithm is shown in FIG. 3for a full level sine wave with 16 drivers. This example has theadvantage that all drivers use substantially the same waveform as shownin FIG. 3.

In a second example, one driver 10 a is always driven and for levels ofthe base signal, which fall within its dynamic range, this is the onlydriver activated. When the level of the signal goes above this, anotherdriver 10 b is switched on such that both now share the load equality(i.e. at changeover the signal to the original driver is halved and thissame half signal is sent to the second driver). When the level exceedsthat which can be accommodated by two drivers, a further driver 10 cwill be switched on such that all three now share the load equally andso on until all drivers are in use. This particular embodiment cansuffer from a problem of significant transients and distortionsoccurring at changeover, but it has the advantage of being particularlyeasy to implement.

In a third example, one driver 10 a is always driven and for levels ofthe base signal which fall within its dynamic range this is the onlydriver activated. When the level of the signal goes above this, anotherdriver 10 b is switch on to add to the first driver 10 a, but the firstdriver 10 a is left saturated such that at the changeover the seconddriver 10 b is at its minimum level. When the level exceeds that whichcan be accommodated by two drivers, a further driver 10 c will beswitched on and so on until all drivers are in use. This algorithm isshown in FIG. 4 for a full level sine wave with 16 drivers. This thirdexample has the advantage of only having signal gradient discontinuitiesat the change over levels—thus reducing unwanted transient switchingproblems.

The type of input signal used by the control processor to control thedrivers is dependent on the frequency. At low frequencies, e.g. below300 Hz, use of linear signals is preferred because the whole panel movesin monophase and at higher frequencies, e.g. greater than 500 Hz, powersignals are preferred because multi-modal resonances are excited in theradiator as described in EP0541646. In the crossover region between 300Hz and 500 Hz, the signals will be partially linear and partially powersignal. The invention applies to any size of loudspeaker. However, atthe low frequency end there may need to be a minimum size to obtain thebenefits of the present invention.

A further improvement is to apply a smoothing function to the controlsignal applied to each newly activated driver, so that the new driver isbrought in in a continuous manner, rather than a step change. An exampleof a suitable smoothing function is a tanh function as shown in FIG. 9for four drivers. As a new driver is added, the signals combine smoothlyuntil the total required level is reached, as illustrated by FIG. 10.

In a fourth example (see FIG. 5) the gain associated with the signal foreach driver is smoothed in the time domain using a moving, shortduration averaging algorithm. This smoothed amplitude signal is used asthe master control to decide the gain of each driver. In essence, eachdriver receives the original waveform but at a level controlled by thesmoothed level of the original waveform.

This example is illustrated in FIG. 5. An input signal is depicted inFIG. 5 as having a rapidly changing level. Controlling the drivers basedon this drive signal could cause non-linear distortion effects and so aboxcar smoothing function is applied to the signal in order to producethe smooth signal depicted in FIG. 5 b. This smooth signal can now beused to determine the number of drivers to be used. In this case theaforementioned algorithm 3 is used and subsequent drivers are activatedas the signal level reaches subsequent respective predetermined levels(see FIG. 5 c). An exponential smoothing function has not been appliedin this instance.

Another feature of the invention is to drive a single panel-formloudspeaker with a number of drivers, possibly identical. The driversare activated to simulate a conventional loudspeaker i.e. more aredriven for low frequency signals than for high frequency signals, inorder to be able to handle the required power. Signals to the driverscould be the same, but a digital filter in front of each driver wouldcontrol the frequency range over which each is driven. A computer couldmanipulate the filter cut off and gain. This would allow the system tobe tailored for different room settings and for the drivers to beswitched on and off according to the absolute power levels required.

In order to further improve the loudspeaker performance, the panel maybe constructed in a tapered form as shown in FIG. 11. The panel has asandwich structure, so that it operates in a region above acousticcoincidence for the greater part of the frequency range. Hence, thecoincidence frequency varies according to panel position. Two skins 14,15 are positioned either side of a cellular core 16. The core may be ahoneycomb or other cellular structure.

1. A multiple input loudspeaker system comprising: at least oneloudspeaker; a plurality of analog drivers for driving the at least oneloudspeaker; and a control processor, responsive to an input signal, forproviding the plurality of analog drivers with a drive signal, saiddrive signal having a time varying signal level and wherein theoperation of each of said drivers is independently controlled inresponse to the amplitude of said input signal level in a givenfrequency range.
 2. A multiple input loudspeaker system as claimed inclaim 1 wherein the plurality of analog drivers drive a plurality ofconventional loudspeakers.
 3. A multiple input loudspeaker system asclaimed in claim 1 wherein the plurality of analog drivers drive apanel-form loudspeaker.
 4. A multiple input loudspeaker system accordingto claim 1, wherein all drivers are driven and the signal level input toeach driver is the lowest integer part of the basic signal levelexpressed as a signed integer plus the number of the drive in question,over the total number of drivers.
 5. A multiple input loudspeaker systemaccording to claim 1, wherein a first driver is activated and drivenuntil the signal level reaches a first predetermined level; wherein asecond driver is activated when the signal level reaches the firstpredetermined level; and wherein subsequent drivers are activated as thesignal level reaches subsequent respective predetermined levels; wherebyall activated drivers share load equally at all activated levels.
 6. Amultiple input loudspeaker system according to claim 1, wherein a firstdriver is driven until the signal level reaches a first predeterminedlevel, wherein a second driver is activated as the signal level reachesthe first predetermined level; wherein subsequent drivers are activatedas the signal level reaches subsequent respective predetermined levels;whereby each newly activated driver takes the load required and allother activated drivers are saturated.
 7. A multiple input loudspeakersystem according to claim 5 wherein the addition of a new driver isachieved in a continuous manner by applying an exponential or othersmoothing function to the signal sent to the drivers.
 8. A multipleinput loudspeaker system according to claim 1 wherein at very low levelsof said drive signal only one driver is activated and at very highlevels of said drive signal all drivers are activated; and wherein thesum of all the driver outputs equals the required signal outputs at alltimes.
 9. A multiple input loudspeaker system according to claim 1wherein a control signal proportional to the amplitude of the drivesignal determines the signal level applied to each driver.
 10. Amultiple input loudspeaker system according to claim 1 wherein a controlsignal proportional to the square of the amplitude of the drive signaldetermines the signal level applied to each driver.
 11. A multiple inputloudspeaker system according to claim 1 wherein the control signal froma controller operates in both linear and power signals, such that at lowfrequencies the controller maintains a linear sum whilst at highfrequencies the controller maintains a power sum.
 12. A loudspeakersystem for translating a master electrical signal into acoustic energy,said system comprising: at least one loudspeaker, a plurality ofloudspeaker driven for translating respective electrical signals intoacoustical movement of said at least one loudspeaker; and a controller,responsive to said master electrical signal, for providing an electricaloutput to each of said drivers, wherein the amplitude of the mastersignal determines the signal supplied to each of said drivers.
 13. Amultiple input loudspeaker system as claimed in claim 12 wherein theplurality of analog driven drive a plurality of conventionalloudspeakers.
 14. A multiple input loudspeaker system as claimed inclaim 12 wherein the plurality of analog drivers drive a panel-formloudspeaker.
 15. A multiple input loudspeaker system according to claim12, wherein all drivers are driven and the signal level input to eachdriver is the lowest integer part of the basic signal level expressed asa signed integer plus the number of the drive in question, over thetotal number of drivers.
 16. A multiple input loudspeaker systemaccording to claim 12, wherein a first driver is activated and drivenuntil the signal level reaches a first predetermined level; wherein asecond driver is activated when the signal level reaches the firstpredetermined level; and wherein subsequent drivers are activated as thesignal level reaches subsequent respective predetermined levels; wherebyall activated drivers share load equally at all activated levels.
 17. Amultiple input loudspeaker system according to claim 16, wherein theaddition of a new driver is achieved in a continuous manner by applyingan exponential or other smoothing function to the signal sent to thedrivers.
 18. A multiple input loudspeaker system according to claim 12,wherein a first driver is driven until the signal level reaches a firstpredetermined level, wherein a second driver is activated as the signallevel reaches the first predetermined level; wherein subsequent driversare activated as the signal level reaches subsequent respectivepredetermined levels; whereby each newly activated driver cakes the loadrequired and all other activated drivers are saturated.
 19. A multipleinput loudspeaker system according to claim 12, wherein at very lowsignal levels of said master electrical signal only one driver isactivated and at very high signal levels of said master electricalsignal all driven are activated; and wherein the sum of all the driveroutputs equals the required signal outputs at all times.
 20. A multipleinput loudspeaker system according to claim 12, wherein said controllerapplies a control signal proportional to the master electrical signal tomaintain the sum of the driver outputs equal to a desired loudspeakeroutput.
 21. A multiple input loudspeaker system according to claim 12,wherein said controller applies a control signal to a suitable squaredtime signal derived from said master electrical signal such that the sumof the loudspeaker acoustic power output is equal to a desired poweroutput.
 22. A multiple input loudspeaker system according to claim 12,wherein the electrical output from said controller operates in bothlinear and power signals, such that at relatively low frequencies thecontroller maintains a linear sum output whilst at relatively highfrequencies the controller maintains a power sum output.